The present invention relates to a belt type transfer device for transferring a toner image on a photosensitive body to a recording medium, for use in an electrophotographic apparatus such as a copier or a printer, and more particularly, to a highly reliable belt-type transfer device having a high transferring operation efficiency and an improved separating efficiency of a recording medium from a photosensitive body, and to an electrophotographic apparatus which improves a performance of a belt-type transfer device.
In a belt type transfer recording medium sheet (sheet) is delivered to a transferring region by an endless belt of an elastic material which is dielectric or has a high electric resistance, and a toner image on a photosensitive body is electrostatically transferred to the sheet. More specifically, the endless belt is rotatably driven and charged by a corona electric charger. The sheet is electrostatically attracted by the belt due to the charged potential so as to be conveyed to the photosensitive body in the transferring region. The photosensitive body carries an electrostatic latent image, and the electrostatic latent image is developed with a toner. In the transferring region, the belt brings the sheet into contact with the photosensitive body. The toner image is electrostatically transferred to the sheet by migration of the charged potential of the belt, that is, an inflow current toward the belt, thus performing the image transferring.
In the transfer device utilizing the endless belt, therefore, the sheet is delivered so as to contact with the belt and the photosensitive body. Previously, the speeds of the belt, the photosensitive body and the sheet where equalized, and the belt was sufficiently charged, in order to ensure attraction of the sheet and good image transfer. During image transfer, a current of 10 .mu.A to 100 .mu.A was caused to flow through the belt to transfer the toner image to the sheet.
After transferring the toner image, however, the sheet sometimes fails to separate from the photosensitive body and adheres to the outer periphery of the photosensitive body because of insufficient contact between the sheet and the belt and the adhesion of the sheet to the photosensitive body, results in a malfunction. Further, the endless belt is electrically charged by friction caused between the belt and the photosensitive body or the sheet during operation, so that the potential of the belt is unstable. In such a state, if the transferring is performed, the electrostatic attraction force necessary for delivery of the sheet is reduced. As a result, the sheet is attracted by the photosensitive body thereby causing a failure of the sheet separation and the image transferring.
To prevent a sheet from adhering to an outer periphery of a photosensitive body in a belt-type transfer device, Japanese Patent Unexamined Publication No. 62-161157 proposes providing an electric charger exclusively used for attraction of the sheet. Moreover, Japanese Patent Unexamined Publication No. 2-95668 proposes a device in which a sheet is curled in a direction of separation of the sheet from a photosensitive drum just entering into before an image transfer section. Additionally, in Japanese Patent Unexamined Publication No. 3-238483, for the purpose of sheet separation, the electric charge of a belt is removed after image transferring.